Focusing Can Be A Problem with Infrared… or Not

Posted on Jul 30, 2018

Today’s Post by Joe Farace

Geek alert: Chromatic aberration is the effect produced by the refraction of different wavelengths of electromagnetic radiation through slightly different angles, resulting in a failure to focus. It causes colored fringes in the images produced by uncorrected lenses.

You may be surprised (or not) to learn that when passing through a lens light waves of different color and wavelengths don’t focus on the same plane. Much like light passing through a prism, the glass elements in a lens produce a rainbow of colors called chromatic aberration as light passes through them. When light passes through the Extra Low Dispersion optics found in some lenses, these specially formulated glass elements minimize the amount of refraction that occurs when forming an image to produce a straighter path for the light.

The infrared difference. The problem for IR shooters is that most lenses can’t focus infrared wavelengths on the same plane as visible light. That’s why the classical advice when shooting infrared images is that it may be necessary to slightly shift the lens’ focus. Typically, the smaller the lens’ aperture and the longer its focal length the greater this shift will be. That’s why lens manufacturers used to place an infrared mark on their lenses (below right) to help you make this shift. But that is seldom possible with newer AF lenses. With my Canon, Lumix or Olympus autofocus lenses, I put the camera in Aperture Preferred mode, set the lens aperture at f/16 or f/11 and just shoot expecting the image to be in focus. And this technique usually usually works for landscape images.

With manual focusing wide-angle lenses like Voigtlander’sSuper Wide-Heliar 15mm f/4.5 Aspherical II lens that was use to make the above image, Hyperfocal distance is my favorite method for focusing when shooting infrared.

Hyperfocal distance is the point of focus where any object that is located between that distance and infinity is in focus. Here’s how it works: You pick an aperture, such as f/16 or f/22, then rotate the focusing ring and setting that aperture opposite the infinity mark. On a 15mm lens like the Voigtlander it produces a depth-of-field from about five inches to infinity, which is more than enough to take care of any focus shift that are caused by invisible infrared light waves.

Life Pixel does a great job with IR conversions and they have done most of the conversions for my Canon DSLRs and all of my Panasonic Lumix G-series cameras.